1. 用于引导限制性定语从句，主要与 such, as, the same连用，其意为“像……的”“凡是……的”“……一类的人(物)”。如：
He’s not such a fool as he looks.
他并不像看上去的那样愚蠢。
He is not such a man as would leave his work half done.
他并不是一个做事半途而废的人。
You must show my wife the same respect as you show me.
你必须对我妻子表现出与对我同样的敬重。
He smokes as expensive a cigarette as he can afford.
他抽一种他买得起的最贵的烟。
2. 用于引导非限制性定语从句，根据情况可位于主句之前或之后，有时也可插在主句中间，其意为“这件事”“这一点”。如( www. yygrammar. com)：
He is late, as is often the case.
他迟到了，这是经常的事。
As is usual with children, they soon got tired.
孩子们一贯如此, 他们很快就厌倦了。
David, as you know, has not been well lately.
大卫最近身体不太好，这你是知道的。
